"You have a teacher."
Translation:Tu magistrum habes.
Because in this sentence, "teacher" is receiving the action directly, making it the direct object of the sentence. Latin has cases, unlike modern Romance languages which lost their cases and use prepositions primarily to indicate such things. In Latin, when a noun is a direct object it is in the "accusative case" and has a different ending than if it is the subject of the sentence (the subject is in the "nominative case"). For a second declension, masculine noun like "magister," the ending is "um." There is a vestige of this in English still--the difference between "he" and "him" is the difference between whether "he" is being used as a subject or an object (though in English "him" can be either an indirect object or a direct object, whereas in Latin the "m" only refers to the direct object).
This is such a ridiculous error on Duolingo's part. It would be unusual to include the "tu" in an actual Latin sentence. You can do it for emphasis, but it certainly isn't necessary and Romans generally didn't do that, any more than it being necessary in Spanish to say "Yo te amo." "Te amo" is completely correct and probably more common (and is actually how you say the same thing in Latin!)
Thanks for the reply! I was probably a little too aggressive in my critique. I wasn't taking into account that this is Beta, and that it is still under construction. As a Latin teacher myself, I was just a bit shocked when I was marked wrong on such a simple translation. With the practically infinite number of possible rearrangements of any given Latin sentence, it must be quite a task to try to develop a program that gets it right. So hats off to your work!